This invention relates to a freeze-stable, allophanate-modified toluene diisocyanurate having an NCO group content of from about 15 to about 42%. These freeze-stable compositions are prepared by reacting toluene diisocyanate and a specific amount of an organic compound which has at least one hydroxyl group in the presence of an allophanate-trimer catalyst or an allophanate-trimer catalyst system.
The trimerization of aromatic isocyanates to form polyisocyanurates is well known in the art. U.S. Pat. Nos. 4,743,627 and 4,382,125 both describe the partial trimerization of polymethylene polyphenylene polyisocyanate (p-MDI), having an average functionality of &gt;2.2, to give stable liquid products having relatively high viscosity at 25.degree. C. (i.e., 2000-100,000 mPa.s).
U.S. Pat. No. 4,284,730 relates to the trimerization of monomeric MDI which has been partially converted to carbodiimide/uretonimine, to give stable liquid polyisocyanurate compositions.
U.S. Pat. No. 5,124,370 describes liquid polyisocyanate mixtures containing isocyanurate groups and having an NCO content of 15 to 30% by weight. These mixtures are obtained by partial trimerization of the isocyanate groups of polyisocyanate mixtures of the diphenylmethane series containing 80 to 100% by weight diisocyanate diphenylmethane isomers and 0 to 20% by weight higher ring compounds of the diisocyanate diphenylmethane series.
The trimerization of toluene diisocyanate in a solvent to make a storage stable liquid is described in both U.S. Pat. No. 4,379,905 and DE 19,523,657. These products are disclosed as being suitable as isocyanate components in two-component polyurethane lacquers. U.S. Pat. No. 4,456,709 describes storage-stable liquid polyisocyanates which have an NCO group content of 36.5 to 45%. These are prepared by mixing 25 to 70 parts of partially trimerized 2,4-TDI with 75 to 30 parts of unmodified 2,4- and/or 2,6-TDI.
Canadian Patent Application 2,113,890 relates to trimer catalyst systems for aliphatic and aromatic isocyanates. The trimer catalyst systems of this earlier application comprise (A) a lithium compound selected from the group consisting of: (i) lithium salts of aliphatic or aromatic monocarboxylic or dicarboxylic acids, (ii) lithium salts of hydroxyl group containing compounds having from 1 to 3 hydroxyl groups per compound, wherein the hydroxyl groups are directly attached to an aromatic ring, and (iii) lithium hydroxide; and (B) an organic compound containing at least one hydroxyl groups. These trimer catalyst systems result in partially trimerized isocyanates which additionally can contain a significant amount of urethane groups.
In accordance with the disclosures of U.S. Pat. No. 4,379,905 and DE 19,523,657, it is necessary that a solvent be present in order to form liquid products. Due to the large quantity of solvent present, these products have restricted uses. In particular, these products are clearly designed for use in coatings applications only.
U.S. Pat. No. 4,456,709 requires pure 2,4-toluene diisocyanate in the first step. The process in this reference results in final products having a relatively narrow NCO content and a restricted distribution of oligomers due to the fact that the trimerization must be completed in the first step of the process.
Advantages of the presently claimed invention include the fact that one is able to use any TDI isomer mixture to obtain freeze-stable products over a broad NCO range. This invention also provides the ability to blend the allophanate-modified TDI trimer products with MDI based isocyanates to obtain freeze-stable products. It is also within the scope of the present invention to prepare freeze-stable polyether and polyester prepolymers from the allophanate-modified TDI trimer products. Until now, it has not been possible to form TDI based trimer products over such a broad range of NCO contents which are freeze-stable over a long period of time. In addition, the use of solvent is not necessary to make the freeze-stable liquid allophanate-modified toluene diisocyanurates of the present invention. In fact, it is preferred that solvents are not used in the present invention.